I read that recipe a while back as well from a Chicago Area Girls Scout's homepage. But I have never tried it. She also had a recipe for fireproofing canvas:

½ c. ammonium phosphate
1 c. ammonium chloride
1 quart water

Mix until dissolved
Soak canvas in solution
Repeat as necessary

But, again, I never tried it. I'm not sure but I suspect that if you wanted to do both, one would fireproof first as the oil in the waterproofing would prevent the fireproofing chemicals from penitrating into the fabric.

Good luck and post your results if you try it.